I think the conditions that lead to root death in higher temps are pretty variable. Even though we've had temps well over 100 (110 was the highest reading on the thermometer in my backyard over the last few weeks), root death isn't inevitable. I've got trees that are still pushing new growth (BC, live oak, zelkova and others), and some that have retreated to summer dormancy (cedar elm). My Japanese maples haven't dropped leaves and show no signs of doing so.
Keeping pots out of the sun and hydrated is a key to preventing leaf drop and root death, I think. I've got my live oak in full sun and it hasn't begun to slow down. I cover the pot with an old white beach towel and wet it down in the morning. Even in that awful sun, the pot remains relatively cool.